I have the 50" ReefBreeders Photon V2 mounted inside my canopy. It is about 10 inches off the waterline. I have used the fixture for a little over 2 years now and also have a 90 gallon mixed reef with mostly acropora on top, montipora in the middle, and zoas, ricordeas, blastos, etc. towards the lower third. I went with the 50" after measuring the brackets my son had mounted inside the canopy and needed the extra 2 inches of fixture length.
My biggest word of caution would be if you mount the fixture inside your canopy, don't be tempted to use anything even close to maximum intensity. I know this from experience! I ramp up to a maximum intensity of about 50% on the blues and white, which happens at 3:00 p.m., and then start ramping down.